Conventional temperature probes, which utilize thermocouples, thermistors, and other electrically conducting components, are often unusable in the presence of electromagnetic fields because of electrical interference problems and field perturbation effects. Metallic components such as lead wires and connectors can cause erroneous temperature readings in the presence of electromagnetic fields, can pick up electrical interference in electrically noisy environments, and can transmit hazardous electrical shocks in high voltage applications.
Optical temperature probes differ from conventional probes in that they contain essentially no metallic or electrically conducting components. Non-metallic temperature probes have applications in and near regions having electromagnetic fields, such as in microwave ovens, motors, transformers, and electrical generators. In addition, nonmetallic probes preclude the possibility of potentially fatal electrical shocks when used to measure temperature inside the human body.
Because of the advantages of nonmetallic temperature probes, several techniques for optically measuring temperature have been proposed and tested. Among these are methods described in the following articles: C. J. Johnson, et al., "A Prototype Liquid Crystal Fiberoptic Probe for Temperature and Power in R. F. Fields", Microwave Journal, Volume 18, No. 8, pp. 55-59, August, 1975; T. Cetas "A Birefringent Crystal Optical Thermometer for Measurements for Electromagnetically Induced Heating, USNC/URSI 1985 Annual Meeting, Boulder, Colo., Oct. 20-23, 1975; D. Christensen, "Temperature Measurement Using Optical Etalons", 1975 Annual Meeting of the Optical Society of America, Houston, Tex., Oct. 15-18, 1975; and "Novel Method for Measuring Transient Surface Temperature with High Spatial and Temporal Resolution", Journal of Applied Physics, Vol. 43, No. 7, p. 3213, July, 1972. Other methods which are currently commercially available include the characterization of fluorescent emission from a fluorescent sensor as described in U.S. Pat. Nos. 4,448,547 and 4,459,044; the measurement of discrete wavelength emissions from an excited semiconductor sensor as described in U.S. Pat. Nos. 4,376,890, 4,539,473; the use of a two-wavelength semiconductor sensor as employed by Mitsubishi Corporation; and the use of a narrow band wavelength source whose optical power is variably absorbed by a semiconductor sensor as disclosed by the present inventor in U.S. Pat. No. 4,136,566.
Except for the fluorescent sensor technique, which measures the time history of the emitted optical power, and the etalon technique, which detects a discrete pass band frequency of a reflecting cavity, the prior methods can be classified generally as "amplitude" techniques. In such "amplitude" techniques, the intensity of the return signal is directly proportional to the temperature. Furthermore, all of the prior methods generally utilize only a small portion of the wavelength spectrum, normally measuring the intensity of no more than two wavelengths of the signal or emission.
A major disadvantage of the prior methods for optically measuring the temperature is that the amplitude techniques are susceptible to inaccuracy caused by drift in the source of intensity, variable optical losses in the transmitting fibers, and other intensity variations unrelated to the sensor temperature. These variations can be minimized by taking a ratio of amplitudes of two wavelengths which interact with the sensor, but a unilateral amplitude drift in either component of the ratio still results in temperature measurement errors. Thus, a need has arisen for a nonmetallic temperature measurement device having a higher degree of accuracy and greater stability with respect to time.